Some forms of vascular abnormality or disease states, such as aneurysms and vascular dissections, affect only portions of a vessel. The term "abnormality," as used herein, refers to any damage or disease state that affects a portion of a vessel wall. An aneurysm, for example, is an area within an artery where the artery wall integrity has become compromised by age, disease or trauma. As a result, blood pressure within the artery causes a portion of the artery wall to bulge or balloon. The portion of the aneurysm attached to the undeformed wall of the parent artery is called the "neck," and the bulbous pouch of the aneurysm is called the "dome." The dome is considerably thinner and weaker than the undeformed parent artery wall, and therefore is much more prone to rupture.
A vascular dissection describes vessel damage in which a portion of a vessel wall delaminates, and a flap of vascular tissue may extend into and partially occlude blood flow in the parent artery. In each of these different types of vascular abnormalities, a portion of a vessel wall is damaged, but the remaining vessel wall is otherwise healthy.
Vascular abnormalities can rupture and result in debilitating injury or death, depending on the size and location of the rupture and the amount of extra-arterial bleeding. For example, an aneurysm located in the brain is called a cerebral aneurysm, and hemorrhagic stroke results when a cerebral aneurysm ruptures. In addition to the risk of stroke, large aneurysms located in certain regions of the brain may result in neurologic problems due to so called "mass effect." This effect is characterized by the enlarged blood filled dome pressing upon important areas of the brain, and may be manifested by symptoms such as seizure, or impaired speech or vision.
Previously known methods for treating cerebral aneurysms include extravascular and endovascular techniques. Extravascular methods require delicate brain surgery to place a clip across the neck of the aneurysm to effectively exclude the dome from blood flow through the undeformed parent artery. Such surgical treatments can be associated with high trauma, long recovery times, incomplete recovery of all neurologic functions, morbidity and mortality associated with open brain surgery. Additionally, aneurysms located in some extremely sensitive areas, such as those surrounding the brain stem, may be inoperable due to the high risk of mortality.
Endovascular techniques, in contrast, treat aneurysms using a microcatheter positioned within the aneurysm or the parent artery. U.S. Pat. No. 5,122,136 to Guglielmi et al. describes one such previously known endovascular technique using a device commonly called a "Guglielmi Detachable Coil" (GDC). A GDC comprises a soft pliable coil made from platinum or platinum alloy that is soldered to a stainless steel coil and push wire. The stainless steel coil and push wire are used to position the platinum coil in the dome of the aneurysm, and position the junction between platinum coil and stainless steel coil near the neck of the aneurysm. A direct current (DC) is applied to the push wire, stainless steel coil and platinum coil to form a thrombogenic mass within the dome and thereby occlude the aneurysm.
By exposing the junction between the platinum coil and its push wire coil to blood and continuing to apply electric current to the push wire, the exposed portion of the stainless steel coil electrolytically dissolves. The remaining portion of the stainless steel coil and push wire then may be withdrawn from the artery, leaving the platinum coil within the dome. Depending on the size of the aneurysm, many such coils (typically from 5 to 20) may need to be placed within the dome to prevent blood from entering the aneurysm. Because pressure on the fragile dome is reduced, the risk of rupture is eliminated or greatly reduced.
Endovascular treatment permits access to vascular lesions through percutaneous introduction of microcatheters through the femoral artery, and therefore involves less patient trauma than an open surgical approach. This often results in a faster recovery and reduced morbidity and mortality. Drawbacks of GDC techniques include patient selection issues--the neck of the aneurysm must be of a sufficient size and orientation to allow coil entry, but prevent coil migration after detachment. Because multiple devices often must be placed directly in the fragile dome, each device introduction risks rupturing the dome due to mechanical trauma induced by the device.
U.S. Pat. No. 5,135,536 to Hillstead describes a stent for treating occlusive vascular disease comprising an expandable wire tube having a reduced diameter for transluminal placement. Once the stent is positioned within a vessel, a balloon catheter is used to expand the stent to support and reinforce the full circumference of the vessel. Such prior art stents typically have high radial strength to resist collapse due to vessel disease. U.S. Pat. No. 5,314,444 to Gianturco describes a stent having similar construction and operation.
Such previously known devices are not suitable for treating vascular abnormalities, such as aneurysms, occurring in highly tortuous vessels. For example, previously known endovascular stents are designed to provide high radial strength when deployed, and therefore generally are too rigid to negotiate the tortuous anatomy of cerebral vessels. In addition, because a stent, once deployed, is often overgrown by thick layer of vessel endothelium, a phenomenon referred to as "neointimal hyperplasia," there is some reduction of the vessel flow area after placement of the stent. Such reduction in flow area may cause an unacceptable reduction of blood flow in cerebral arteries. Some researchers believe that the higher the percent coverage of an artery by a stent, the more hyperplasia will occur.
As a result of the drawbacks of previously known endovascular techniques, it is desirable to find an alternative solution for treating vessels. In Wakhloo et al., "Self-Expanding and Balloon-Expandable Stents in the Treatment of Carotid Aneurysms: An Experimental Study in a Canine Model," Am. J. Neuroradiology, 15:493-502 (1994), the authors describe the feasibility of placing a stent across a portion of the neck of an aneurysm to alter the hemodynamics and therefore induce spontaneous clotting of stagnant blood within the dome. Those authors further postulated that the struts of the stent covering the neck of the aneurysm may provide a lattice for the growth of new endothelial cells across the neck, permanently excluding it from blood flow through the parent artery. Shrinking the aneurysm and resorption of blood within the aneurysm are expected to follow, thus preventing long-term mass effect problems.
In view of the foregoing, it would be desirable to provide methods and apparatus to enable a stent to be atraumatically and transluminally inserted into highly tortuous vessels, such as the cerebral arteries.
It further would be desirable to provide methods and apparatus for deploying a stent that spans a portion of a vessel to provide a lattice for the growth of new endothelial cells across the portion.
It also would be desirable to provide methods and apparatus comprising a stent having sufficient radial strength to resist downstream migration within the parent artery, but which is less subject to narrowing of the vessel flow area.